1. Field of the Invention
This invention provides a device to remove and store tritium from a gaseous medium, and a method for manufacturing the device. It specifically provides a device which may be incorporated in a fuel rod of a nuclear reactor to minimize release of tritium to the reactor coolant.
2. Description of the Prior Art
The operation of a nuclear reactor necessarily forms tritium. As a product of ternary fission, which is typically the largest source of tritium, tritium is formed within the solid matrix of uranium containing pellets and other fuels, typically encased in metal tubes or cladding. Most water reactors utilize fuel cladding of zirconium alloy, known more commonly as Zircaloy, and a typical commercial reactor includes thousands of such rods. The properties of typical zirconium alloys are defined in ASTM Standard B 353-71, "Wrought Zirconium and Zirconium Alloy Seamless and Welded Tubes for Nuclear Service." After formation of tritium in the solid fuel pellet matrix, the gaseous tritium may diffuse through the pellet matrix and into the void volume between the fuel pellets and fuel cladding, as do a variety of other fission product gases. These fission product gases are then free to migrate throughout the fuel rod, and contribute to a pressure buildup within the cladding. The tritium, and other fission product gases, circulate inside the fuel rod due to convection. A typical fuel rod includes a plenum area at top of the rod, where, due to the free volume, these gases tend to collect.
Although the radioactivity emitted by tritium is a weak beta emission, and although it has a relatively short biological half-life (ten days), tritium has a relatively long radioactive half-life (twelve years). Also, tritium will readily diffuse through most materials, including materials such as zirconium, alloys of zirconium, and stainless steel, which are typically used as fuel rod cladding. Because pressurized water reactors in operation today utilize boric acid in the coolant for power level control, tritium is also formed within the reactor coolant itself. Once tritium reacts with water to form HTO, it is technically difficult and very costly to separate.
Regulatory authorities have therefore placed stringent restrictions on allowable releases of tritium to the environment. One way to lower the tritium inventory in the reactor coolant, and hence the amount of tritium which may be discharged to the environment, it to provide a means within each fuel rod to specifically collect and store the tritium produced within the fuel pellets which diffuses into the void volume. This invention provides such means, which further are easily removable from the fuel rod during reprocessing. As tritium is widely used as a tracer element and in the medical and other fields, being able to simply and less expensively recover the tritium, as compared to recovery from an aqueous solution, is a further benefit provided by this invention. The device disclosed herein also may be utilized in other functions within a nuclear plant, as well as in other applications where it is desirable to remove tritium from a gaseous medium.
Although many systems and modes of operation have been used and proposed to control tritium subsequent to its entering the reactor coolant, in accordance with this invention tritium is specifically collected and controlled within the fuel rod itself. This invention, in the preferred embodiment, does so by means of a device consisting of an inner core of zirconium or alloys of zirconium, covered on all surfaces with an adherent layer of nickel, which nickel layer acts as a selective and protective window for the passage of tritium. At reactor operating temperatures, the layer of nickel is generally unreactive to species in the fuel rod environment, including any high temperature moisture present. The nickel layer, however, is selectively permeable to tritium, also allowing passage of such atomically small and available isotopes as hydrogen and deuterium. Once through the adherent nickel layer, the tritium reacts with the inner core of zirconium alloy to form a solid solution or hydride, and is fixed within the zirconium alloy matrix until such time as it is desirable to remove the tritium.
Other devices have been disclosed which may perform a somewhat similar function, although of different design and without the tritium selectivity provided by the device of the instant invention. A United States patent issued to L. N. Grossman, U.S. Pat. No. 3,742,367, June 1973, discloses a non-destructive detection process for nuclear fuel rods. The Grossman patent provides, in part, a device consisting of a homogeneous alloy of titanium, zirconium, and nickel, as differentiated from the layered window of nickel over a zirconium alloy core of this invention. An amount of the alloy of the Grossman patent is placed in the fuel rod during assembly. The assembled rod is then heated prior to installation in the reactor, to vaporize moisture, and free from the fuel pellet matrix gases such as hydrogen, oxygen, nitrogen, carbon monoxide, and carbon dioxide which react with the alloy. The alloy within the rod is then examined by neutron radiography to detect metallic hydrides prior to putting the fuel rod into operation in a reactor. Detection of moisture provides an indication that sufficient heating of the fuel has occurred to remove moisture from the fuel pellets. This reaction of the named elements and compounds with the homogeneous alloy is designed to occur to eliminate subsequent embrittlement and induced stresses in the cladding during reactor operation. Since the homogeneous alloy of the Grossman patent is reactive with hydrogen, it should also be reactive with tritium released during reactor operation.
However, it is seen that significant differences exist between this invention and the teachings of the Grossman patent. Most notably, these distinctions include differences in elemental composition and in the methods of joining the elements. The prior art device consists of titanium, zirconium, and nickel, compared to a nickel coated zirconium alloy of this invention. More important, the prior art forms these elements into a homogeneous alloy, with the reactions taking place on the surface and within the alloy. This invention, on the other hand, provides a two-layered composite device, containing zirconium or zirconium alloy as an internal core and a layer of nickel on the exterior. The device disclosed herein is much more selective as to what will pass through the nickel layer or window and react inside the device with zirconium alloy. Additionally, the prior art alloy is used to remove moisture and other impurity gases from a fuel rod prior to reactor operation, whereas the device disclosed herein performs its function subsequent to reactor startup and during the life of the fuel rod. The device disclosed herein further has significant benefits in terms of tritium recovery and separation subsequent to reactor operation.